1. Field of the Invention
The present invention relates generally to broadband infrared windows, and, more particularly, to a low cost process for manufacturing such windows.
2. Description of Related Art
Recent efforts have been undertaken to develop a vision enhancement system for automotive and other high volume applications. Such systems employ infrared (IR) detectors and are used, for example, to provide an early warning of close proximity to other vehicles.
Infrared emitter and detector assemblies commonly employ a window which protects the infrared components while permitting the transmission of infrared radiation. Commonly, II-VI materials, such as zinc sulfide (ZnS), zinc selenide (ZnSe), and cadmium telluride (CdTe), have been employed as infrared window materials.
ZnS windows were produced by hot pressing of ceramic powders years ago by Kodak and others; see, e.g., U.S. Pat. No. 3,131,238, issued Apr. 28, 1964. The development of a chemical vapor deposition (CVD) process for fabricating ZnS largely replaced the prior method of making ZnS for military applications where cost was not an important consideration. The CVD Zns could be fabricated in larger, flat or curved pieces to conform to aircraft shape requirements. However, there was no improvement in mechanical properties with the CVD process. In fact, when CVD ZnS was annealed, it became more transmissive in the optically visible region, but the ZnS also became much softer. It is believed that the removal of the high temperature (hexagonal) wurtzite phase by the heat treatment is responsible for the increase in IR transmission and that the growth of large grains caused the material to soften. For a discussion of ZnS formation by hot pressing and by CVD, see J. A. Savage, Infrared Optical Materials and Their Antireflection Coatings, Adam Hilger LTD., Bristol and Boston (1985), pp. 95-111.
The CVD process is very slow and not easily amenable to high volume production, and typically requires a week to make a CVD run. The as-formed CVD ZnS has a yellow color due to the presence of absorbing defects. But most important is the intrinsic high cost of CVD fabrication which makes IR windows produced by this method prohibitively expensive for high volume markets.
The use of gallium sulfide for hardening zinc sulfide was investigated by J. Zhang et al, "Solid-State Phase Equilibria in the ZnS-Ga.sub.2 S.sub.3 System", Journal of the American Ceramic Society, Vol. 73, No. [6], pp. 1544-1547 (1990). It was found that ZnS-Ga.sub.2 S.sub.3 solid solution yielded more than a 50% increase in hardness and fracture toughness. A model was developed that correlated porosity and second phase material (ZnGa.sub.2 S.sub.4, zinc thiogallate) with transmission in the infrared region. The model predicted that when porosity approached zero volume percent and precipate sizes were smaller than one micrometer, the transmission properties of zinc sulfide would not be altered.
The work of Zhang et al was limited to the addition of gallium sulfide in the bulk zinc sulfide material. They published additional work on the phase equilibria of the gallium and zinc sulfide binary system to establish a means for forming zinc thiogallate as a second phase. The solid solution region for gallium in zinc sulfide was found to decrease with lower temperatures below the cubic to hexagonal phase transformation temperature at 1,025.degree. C. Thus, at a lower temperature, the gallium would be expected to precipitate out as a second phase, zinc thiogallate (ZnGa.sub.2 S.sub.4) as described by W. W. Chen et al, "Experimental and Theoretical Studies of Second-Phase Scattering in IR Transmitting ZnS-Based Windows", Proceedings of SPIE, San Diego (1991).
A low cost IR window is a mandatory requirement in order to be able to market an affordable vision enhancement system for automotive and other high volume applications. The cost of the IR window is a major factor in the viability of offering an IR device for general use. The potential safety benefits to the driving public are enormous. Many lives could be saved and needless destruction of automobiles could be prevented if such a device were available at a reasonable cost.
Accordingly, a need exists for the production of a low cost IR window.